Fm isolation transformer coupling to dual frequency transmitter antenna



p 1968 R. M. SILLIMAN FM ISOLATION TRANSFORMER COUPLING TO DUAL FREQUENCY TRANSMITTER ANTENNA Filed March 23. 1966 JOA TO ANTENNA INVENTOR ROBERT M. SILL! MAN Illfllffli FIGZ ATTORNEYS United States Patent FM ISOLATION TRANSFORMER COUPLING TO DUAL FREQUENCY TRANSMITTER ANTENNA Robert M. Silliman, Silver Spring, Md., assignor to Electronics Research, Inc., Evansville, Ind., a corporation of Indiana Filed Mar. 23, 1966, Ser. No. 536,717 2 Claims. (Cl. 343208) ABSTRACT OF THE DISCLOSURE An isolation transformer is disclosed for coupling between a transmitter and an antenna to operate at two differing frequencies such as used in AM-FM transmission. Structure including a grounded metal housing with a portion insulated envelops internal coupling elements ineluding a tuned R-F transformer arrangement coupling the antenna and transmitter radio frequency energy between two electrically isolated coaxial lines.

This invention relates to high power radio transmitter feed lines and, more particularly, it relates to isolation transformers coupling an FM transmitter to a dual frequency antenna.

In order to use high powered transmitting antennas simultaneously for two different signals of different frequency such as AM-FM transmission, precautions need to be taken to properly convey the signals without interaction.

It is therefore an object of this invention to provide an improved isolation transformer for feeding FM signals to a dual purpose AM-FM transmitter antenna without significant losses of or interaction between the two signals.

There is thus provided by the invention an antenna coupling transformer for conveying FM power from a transmission line across the base insulator of a transmitting tower used jointly as an AM and FM radiator to prevent losses of AM signals and to assure proper FM feed. The transformer is mounted within a grounded conductive housing which encompasses the coupling and tuning structure.

The structural configuration, further objectives and features of advantage are described throughout the following specification with reference to the accompanying drawing, wherein:

FIGURE 1 is a pictorial view of an opened housing containing the isolation transformer assembly afforded by the invention;

FIGURE 2 is a diagrammatic view of the structural configuration of a transformer embodiment constructed in accordance with the teachings of this invention; and

FIGURE 3 is a schematic diagram of a further embodiment of the invention.

The input line to the isolation transformer comprises a standard 50 ohm coaxial transmission line 10, which is coupled at an adapter joint 11 extending from a conductive enclosure 15 which may be hermetically sealed by covering the exposed opening face 14. Similarly, output line 20 may be coupled to connector joint 21.

The input line comprises a coaxial conductor 12 with an inner wire 16 extending into the interior of the housing 15 and an electrically conductive bond 17 between the outer (grounded) conductor and the conductive housing 15. However, the outer conductor of the output line 22 is insulated from the conductive housing 15 by means of the insulating end member 23. This prevents an antenna tower or line connected to the outer conduct-or of the output line 22 from being grounded, and the transformer by way of the circuits inside the housing 15 presents a very high impedance to low radio frequencies in the AM audio broadcast bands of say 540 to 1600 kilocycles. Of course,

should the transformer be reversed, the insulating feature will appear at the transmitter rather than at the antenna. Accordingly, little loss of energy from an AM transmitter supplied to a common FM-AM antenna structure is permitted by this structure.

On the other hand, however, energy from an FM transmitter coupled to input line 12 is passed through the transformer structure to the antenna coupled to output line 22 with very little loss. This results by mutual inductive coupling between the respective input and output circuit lines 30, 31, which comprise resonant circuits tuned to a frequency such as 100 megacycles near that of the FM carrier to be passed to the antenna.

The lines 30 and 31 are tuned by respective adjustable capacitor plate pairs 32 and 33 located between the ends of the loop conductors to match the characteristic impedances of the lines at the frequency being transmitted through the isolation transformer array. The self inductance of the loops 30, 31 with the lumped capacity component of plate pairs 32, 33 therefore constitutes the respective tuned circuit parameters which develop the energy coupled from input line 12 through output line 22 by the mutual impedance between the loops 30 and 31.

Even with matched termination impedances, because of the high power of specific frequency at the antenna structure coupled to output line 22, it is possible to set up an undesirable FM voltage across the isolation transformer which will dissipate power. For this reason the embodiment of FIGURE 3 provides for balancing capacitances 40-43 coupled between each capacitor plate and the grounded housing. The values of the capacitances 40-43 may be calculated to achieve a balanced condition minimizing the losses from undesired voltage modes. Since each of these capacitances is small, it may comprise a built-in distributed capacitance between the respective line and the housing. For example, capacitance 40 may comprise at least in part the distributed capacity between line 31A and the housing, which is established by spacing parameters within the housing. In this manner standing wave ratios of less than 1.05 to 1 at a specified frequency can be provided when the transformer is terminated in a matched ohm load.

The housing which is hermetically sealed by cover plate 40, and the coaxial lines 12, 22, etc. are hermetically sealed to provide for operation at a pressure of twenty pounds per square inch when gas is introduced into the unit by Way of a closed pressure system.

Having therefore described the nature of the invention and its mode of operation by reference to a detailed preferred embodiment, I therefore claim those features defined in the appended claims which are believed to define the spirit and scope of the invention.

What is claimed is:

1. An isolation transformer assembly for coupling high frequency FM power into a transmitting antenna used jointly for radiating low frequency AM and FM comprising in combination an electrically conductive housing with an insulating end portion, an input two wire coaxial transmission line adapted for coupling to an FM transmitter with an outer conductor electrically connected to and mounted on said conductive housing and an inner conductor passing into the interior of said housing, an output two wire coaxial transmission line adapted for coupling to said transmitting antenna having an outer conductor mounted on the insulating end portion of said housing and an inher conductor passing into the interior of said housing, a first resonant loop having a resonant frequency near the FM band provided by the transmitter and located within said housing comprising a pair of conductors coupled to the input transmission line and coupled near their ends by a pair of capacitor plates which are adjustably spaced from each other, a second resonant loop of similar con- 3 v struction having substantially the same resonant frequency and inductively coupled within said housing with the first loop with two conductors coupled to the output transmission line, whereby a lower AM frequency coupled to the output transmission line is isolated from the input transmission line.

2. An isolation transformer as defined in claim 1, having capacitances of such values from each of the four capacitor plates to the housing of such relative values that undesirable voltage modes between the input and output nulled.

References Cited UNITED STATES PATENTS ROBERT L. GRIFFIN, Primary Examiner.

transmission lines where mounted on said housing are 10 B. V. SAFOUREK, Assistant Examiner. 

